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Abstract:

A capacity and density enhancement circuit for a sub-threshold memory
unit array which can decrease the drain current in the bit lines and
enhance the pull-up capability of memory cells. The capacity and density
enhancement circuit is composed of a first enhancement transistor, a
second enhancement transistor, a first mask transmission gate, a second
mask transmission gate, a first logic memory capacitor and a second logic
memory capacitor.

Claims:

1. A capacity and density enhancing circuit for a sub-threshold memory
cell, comprising: a first enhanced transistor, a second enhanced
transistor, a first shielded transmission gate, a second shielded
transmission gate, a first logical storage capacitor, and a second
logical storage capacitor, wherein, a source end of the first enhanced
transistor is connected with a source end of the second enhanced
transistor and is connected to a supply voltage, a drain end of the first
enhanced transistor is connected with an input/output end of the first
shielded transmission gate and serves as one end of a bit line of a
memory cell array, a drain end of the second enhanced transistor is
connected with an input/output end of the second shielded transmission
gate and serves as a NOT end of the bit line of the memory cell array, a
body end of the first enhanced transistor is connected with a local grid
end, a grid end of the first enhanced transistor is connected with one
end of the first logical storage capacitor and is connected to the
output/input end of the first shielded transmission gate, another end of
the first logical storage capacitor is grounded, a body end of the second
enhanced transistor is connected with the local grid end, a grid end of
the second enhanced transistor is connected with one end of the second
logical storage capacitor and connected to the output/input end of the
second shielded transmission gate, another end of the second logical
storage capacitor is grounded, a control end of the first shielded
transmission gate and second shielded transmission gate is used for input
of an enhanced control signal, a complementary control end of the first
shielded transmission gate and second shielded transmission gate is used
for a NOT input of the enhanced control signal.

2. The capacity and density enhancing circuit for a sub-threshold memory
cell array according to claim 1, wherein, the first shielded transmission
gate consists of a first PMOS transistor and a first NMOS transistor, a
body end of the first PMOS transistor is connected with the local grid
end and serves as the control end of the first shielded transmission
gate, a body end of the first NMOS transistor is connected with the local
grid end and serves as the complementary control end of the first
shielded transmission gate, a source end of the first PMOS transistor is
connected with a drain end of the first NMOS transistor and serves as the
input/output end or output/input end of the first shielded transmission
gate, and a drain end of the first PMOS transistor is connected with a
source end of the first NMOS transistor and serves as the output/input
end or input/output end of the first shielded transmission gate.

3. The capacity and density enhancing circuit for sub-threshold memory
cell array according to claim 1, wherein the second shielded transmission
gate consists of a second PMOS transistor and a second NMOS transistor, a
body end of the second PMOS transistor is connected with the local grid
end and serves as the control end of the second shielded transmission
gate, a body end of the second NMOS transistor is connected with the
local grid end and serves as the complementary control end of the second
shielded transmission gate, a source end of the second PMOS transistor is
connected with a drain end of the second NMOS transistor and serves as
the input/output end or output/input end of the second shielded
transmission gate, and a drain end of the second PMOS transistor is
connected with the source end of the second NMOS transistor and serves as
the output/input end or input/output end of the second shielded
transmission gate.

Description:

PRIORITY CLAIM

[0001] The present application is a National Phase entry of PCT
Application No. PCT/CN2009/073248, filed Aug. 18, 2009, the disclosure of
which is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0002] The present invention is mainly designed to increase the number of
memory cells that can be connected in series on the bit lines of a
sub-threshold memory cell array, and thereby enhance the capacity and
density of the memory cell array. The circuit mainly operates in the
sub-threshold area (the supply voltage is lower than the threshold
voltage of the transistors). The bit line is shielded from the drain
current of matching transistors for unselected memory cells under the
control of the shielding logic, so that the bit line logic is only
subject to the power-up current of selected memory cells; in that way,
the effect of the power-up current of selected memory cells to the
oscillation amplitude of the bit line is enhanced, and therefore it is
possible to implement memory cell arrays in large scales.

BACKGROUND OF THE INVENTION

[0003] A memory cell array is an important component in modern digital
systems, and is often a bottleneck of power consumption in system design.
As the demand for portable devices increases in the market, the
requirement for technologies to reduce the power consumption of memory
cell arrays becomes higher and higher. Sub-threshold design is a focus in
extremely low power design nowadays. By reducing the supply voltage (Vdd)
entering into the sub-threshold area of the circuit--Vdd lower than the
threshold voltage (Vth), the system can operate in the linear range of
the circuit, and thereby the dynamic and static power consumption of the
system can be reduced significantly. The design of sub-threshold memory
cell arrays further highlights the advantage of sub-threshold design in
low-power consumption. However, in the actual implementation process, the
design introduces a series of problems: 1) the power-up current/shutdown
current ratio (Ion/Ioff) is low--in conventional designs, the power-up
current/shutdown current ratio (Ion/Ioff) is approximately 107,
whereas in sub-threshold design, the Ion/Ioff ratio is only
103-104; 2) the number of memory cells that can be connected in
series on a bit line is limited, and therefore the capacity of a memory
cell array is limited, and the area consumption is high; 3) in the read
cycle, the oscillation amplitude of a bit line is low, and the detection
tolerance of a sensitive amplifier is low; 4) the performance is
susceptible to process deviations, etc. The key point in the problem is
that the effect of drain current in the matching transistors for
unselected logical memory cells on the bit line in the sub-threshold area
is much more severe than the effect in the super-threshold area under the
corresponding conditions. In addition, the effect will be further
aggravated if there are process deviations. If there is not enough
redundancy (the number of memory cells connected in series on the bit
line is greater than a threshold), the power-up current of a selected
memory cell may be interfered by the drain current accumulated in the
unselected memory cells; as a consequence, the circuit can't recognize
the correct logic subsequently, which will result in read and write
failure of the memory cell (FIG. 1). In view of the process deviations
and the bias voltage of the follow-up sensitive amplifier, the number of
memory cells on a bit line will be further limited. At present, there are
two trends in the design of sub-threshold memory cell arrays: 1)
calculate the proportional relation of Ion/Ioff of the transistors within
different process corners, and control the number of memory cells on the
same bit line strictly; 2) introduce drain current compensation logic in
the memory cell. However, the memory cell arrays are high-capacity
logical units and have high requirements for design density. Both methods
described above can't effectively solve the problem of large chip area
consumption of memory cell array.

SUMMARY OF THE INVENTION

[0004] To solve the key problems in the existing sub-threshold memory
circuits, on the basis of the characteristics of sub-threshold circuits,
the present invention provides a capacity and density enhancing circuit
for sub-threshold memory cell arrays, which can decrease the drain
current in the bit lines and enhance the pull-up capability of the memory
cells.

The Present Invention Employs the Following Technical Solution:

[0005] A capacity and density enhancing circuit for a sub-threshold memory
cell array, comprising a first enhanced transistor, a second enhanced
transistor, a first shielded transmission gate, a second shielded
transmission gate, a first logical storage capacitor and a second logical
storage capacitor, wherein the source end of the first enhanced
transistor is connected with the source end of the second enhanced
transistor and is connected to the supply voltage, the drain end of the
first enhanced transistor is connected with the input/output end of the
first shielded transmission gate and serves as one end of bit line of a
memory cell array, the drain terminal of the second enhanced transistor
is connected with the input/output end of the second shielded
transmission gate and serves as the NOT end of bit line of the memory
cell array, the body end of the first enhanced transistor is connected
with the local grid end, the grid end of the first enhanced transistor is
connected with one end of the first logical storage capacitor and
connected to the output/input end of the first shielded transmission
gate, the other end of the first logical storage capacitor is grounded,
the body end of the second enhanced transistor is connected with the
local grid end, the grid end of the second enhanced transistor is
connected with one end of the second logical storage capacitor and
connected to the output/input end of the second shielded transmission
gate, the other end of the second logical storage capacitor is grounded,
the control end of the first shielded transmission gate and second
shielded transmission gate is used for input of enhanced control signal
respectively, and the complementary control end of the first shielded
transmission gate and second shielded transmission gate is used for NOT
input of enhanced control signal respectively.

[0006] The present invention operates in the sub-threshold area, and can
shield the effect of drain current in a bit line to the bit line logic
automatically, therefore, it can increase the capacity and density of a
memory cell array by several times, and is especially suitable for
high-capacity and high-density sub-threshold memory cell arrays with
extremely low power consumption.

Beneficial Effects:

[0007] (1) The logical voltage values on the complementary bit lines in an
initial state are recorded and stored in the first logical storage
capacitor and the second logical storage capacitor, respectively. In the
read and write operation, the logical voltage values of complementary bit
lines stored in the first logical storage capacitor and second logical
storage capacitor are taken as the ON/OFF control signals for the first
enhanced transistor and the second enhanced transistor, and the charging
of the complementary bit lines by the supply voltage Vdd is controlled
dynamically, so as to reduce the effect of drain current in unselected
memory cells on the bit line in the sub-threshold area to the bit line
logic.

[0008] (2) Memory cells working in sub-threshold area usually have poor
pull-up capability. The circuit can regulate the pull-up driving power
automatically according to the actual information stored in the memory
cell array. Thus, the high logic electrical level on the bit lines can be
recognized correctly even if there is no sensitive amplifier.

[0009] (3) The body end of the transistor is connected directly with the
local grid end of the transistor. With the unique connection mode in the
sub-threshold area: 1) when the transistor is in ON state, the threshold
voltage is low, and the power-up current is high; 2) when the transistor
is in OFF state, the threshold voltage is equal to the threshold voltage
in the conventional connection mode, and therefore the electrical
properties (e.g., shutdown current) are the same as those in the
conventional connection mode. In this design mode, the Ion/Ioff ratio of
the present invention is improved, and thereby the adverse effect of
drain current to the performance of circuit in sub-threshold area is
reduced.

[0010] (4) The chip area consumption of the present invention is low.

[0011] (5) The sequential control signal is simple, and therefore the
errors introduced by high delay deviations of the circuits in the
sub-threshold area are avoided.

[0012] (5) The present invention can be applied to a sub-threshold circuit
design, and has the remarkable feature of extremely low power
consumption.

[0013] (6) The present invention has a high tolerance to process
deviations. As indicated in the result of a Monte Carlo analysis, the
present invention can reduce the drain current in memory cells on a bit
line reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 shows a logic block diagram of a sub-threshold memory cell
array and a schematic diagram of the influence of drain current on the
bit line in the worst case;

[0015] FIG. 2 shows structural diagrams of the capacity and density
enhancing circuit for a sub-threshold memory cell array, wherein the FIG.
2(a) is a structural diagram of the first shielded transmission gate
circuit in the present invention, and FIG. 2(b) is a structural diagram
of the second shielded transmission gate circuit in the present
invention;

[0016] FIG. 3 shows the structure of a pre-charge/equalization circuit.
The time sequence of the enhancing circuit in the present invention
should work with the time sequence of a pre-charge/equalization circuit;

[0017] FIG. 4 shows the control sequence and waveform diagram of an
enhancing circuit and pre-charge/equilibration circuit that work
together;

[0018] FIG. 5 shows a comparison between the waveform diagram of
complementary bit lines that employ the enhancing circuit and the
waveform diagram of complementary bit lines that don't employ the
enhancing circuit, and the control signals for the bit lines;

[0019] FIG. 6 shows the voltage signal on a bit line that employs the
circuit in the present invention in the "1" reading operation and the
voltage signal on a bit line that doesn't employ the circuit in the
present invention in the "1" reading operation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] A capacity and density enhancing circuit for sub-threshold memory
cell, as shown in FIG. 2, comprises a first enhanced transistor P1, a
second enhanced transistor P2, a first shielded transmission gate T1, a
second shielded transmission gate T2, a first logical storage capacitor
CAP1, and a second logical storage capacitor CAP2, wherein, the source
end of the first enhanced transistor P1 is connected with the source end
of the second enhanced transistor P2 and is connected to the supply
voltage, the drain end of the first enhanced transistor P1 is connected
with the input/output end of the first shielded transmission gate T1 and
serves as one end of bit line of a memory cell array, the drain end of
the second enhanced transistor P2 is connected with the input/output end
of the second shielded transmission gate T2 and serves as the NOT end of
bit line of the memory cell array, the body end of the first enhanced
transistor P1 is connected with the local grid end, the grid end of the
first enhanced transistor P1 is connected with one end of the first
logical storage capacitor CAP1 and is connected to the output/input end
of the first shielded transmission gate T1, the other end of the first
logical storage capacitor CAP1 is grounded, the body end of the second
enhanced transistor P2 is connected with the local grid end, the grid end
of the second enhanced transistor P2 is connected with one end of the
second logical storage capacitor CAP2 and is connected to the
output/input end of the second shielded transmission gate T2, the other
end of the second logical storage capacitor CAP2 is grounded, the control
end ( comp) of the first shielded transmission gate T1 and the second
shielded transmission gate T2 is used for input of enhanced control
signal respectively, the complementary control end (comp) of the first
shielded transmission gate T1 and the second shielded transmission gate
T2 is used for NOT input of enhanced control signal.

[0021] The first shielded transmission gate T1 consists of a first PMOS
transistor TP1 and a first NMOS transistor TN1, wherein, the body end of
the first PMOS transistor TP1 is connected with the local grid end and
serves as the control end ( comp) of the first shielded transmission gate
T1, the body end of the first NMOS transistor TN1 is connected with the
local grid end and serves as the complementary control end (comp) of the
first shielded transmission gate T1, the source end of the first PMOS
transistor TP1 is connected with the drain end of the first NMOS
transistor TN1 and serves as the input/output end or output/input end of
the first shielded transmission gate T1, the drain end of the first PMOS
transistor TP1 is connected with the source end of the first NMOS
transistor TN1 and serves as the output/input end or input/output end of
the first shielded transmission gate T1.

[0022] The second shielded transmission gate T2 consists of a second PMOS
transistor TP2 and a second NMOS transistor TN2, wherein, the body end of
the second PMOS transistor TP2 is connected with the local grid end and
serves as the control end ( comp) of the second shielded transmission
gate T2, the body end of the second NMOS transistor TN2 is connected with
the local grid end and serves as the complementary control end (comp) of
the second shielded transmission gate T2, the source end of the second
PMOS transistor TP2 is connected with the drain end of the second NMOS
transistor TN2 and serves as the input/output end or output/input end of
the second shielded transmission gate T2, and the drain end of the second
PMOS transistor TP2 is connected with the source end of the second NMOS
transistor TN2 and serves as the output/input end or input/output end of
the second shielded transmission gate T2.

[0023] The present invention comprises enhanced PMOS transistors P1 and
P2, shielded transmission gates T1 and T2, and logical storage capacitors
CAP1 and CAP2. The source ends of the enhanced transistors P1 and P2 are
directly connected to the supply voltage respectively, and the drain ends
of the transistors are connected with a pair of complementary bit lines,
respectively. The storage capacitors are directly connected with the grid
ends of the enhanced transistors P1 and P2 and are connected to one end
of shielded transmission gates T1 and T2, respectively. The other ends of
shielded transmission gates T1 and T2 are connected with a pair of
complementary bit lines, respectively. The time sequence of the entire
enhanced logic should be used in conjunction with the
pre-charge/equalization signal eq of pre-charge logic (FIG. 3) that is
used in conventional memory cell arrays.

[0024] FIG. 4 shows the sequential relationship between the control signal
of the enhancing circuit and the pre-charge logic. In the cycle TA, the
enhanced control signal comp is enabled, and therefore the transmission
gates T1/T2 are turned on. Thus, the logical information of the
complementary bit line pair in initial state is stored into the
capacitors CAP1 and CAP2. In this cycle, since the information stored in
the memory cells on the bit lines is different among the memory cells,
the voltage information of the complementary bit line pair is also
different and the specific values are between supply voltage Vdd and GND.
In the cycle TB, the enhanced control signal comp is invalid, and the
system enters into a pre-charge and equilibration cycle. The pair of
complementary bit lines is pre-charged to the supply voltage Vdd. In
addition, since the period is short and the shielded transmission gates
T1 and T2 are in an OFF state, the effect of a logical voltage change on
the bit lines to the information stored in the capacitors is not severe.
It should be noted, in theory, when the system transits from the cycle TA
to the cycle TB, at the end of storage of logical information of the bit
lines in the capacitors at the rising edge of comp, the logical level of
the complementary bit lines can be increased directly following the
falling edge of eq and the memory cell array is enabled to enter into the
pre-charge cycle, however, in view of the demand for tolerance to process
deviations, some margin is reserved in the sequential circuit in the
actual design of the present invention between the rising edge of comp
and the falling edge of eq. The rising edge of eq indicates the start of
cycle TC. In the cycle TC, the memory cell array enters into a read/write
cycle. At this point, the enhancing circuit can regulate the charging of
the complementary bit line pair by the supply voltage Vdd according to
the charged potential of the capacitors.

[0025] In reading operation, the matching transistors for selected memory
cells are controlled to turn on (the word line of the memory cells is
"1", WL=1), the information stored in the memory cells is transmitted on
the complementary bit line pair. Thus, theoretically, the potential of
the bit line should be full zero, and the potential of the NOT bit line
should be the entire supply voltage Vdd. However, due to the existence of
capacitors on the bit line and the drain current accumulated in the
unselected memory cells (the word line of the memory cells is "1", WL=0),
the voltage of the bit line (BL) is greater than potential 0, while the
voltage of the NOT BL is lower than Vdd (FIG. 1).

[0026] FIG. 5 shows the simulation test result of the effect of drain
current to the oscillation amplitude of bit lines with 256 memory cells
connected in series at 400 mV in the worst case (the selected memory
cells store signal "1", while the unselected memory cells store signal
"0", i.e., the drain current in the bit lines is the highest). It is seen
from the figure, on the complementary bit line pair that doesn't employ
the enhancing circuit, the voltage difference is very low (18.4 mV) due
to the effect of drain current. After the enhancing circuit is employed
on the complementary bit line pair, the voltage difference is increased
from 18.4 mV to 151.2 mV. Thus, the follow-up sensitive amplifier has
enough detection margin to detect correct read data. It is proven in
tests that a sub-threshold memory cell array with 512 memory cells
connected in series can still operate normally in the worst case with the
circuit provided in the present invention. That is to say, the memory
cell arrays that employ the circuit provided in the present invention can
support more memory cells. The sub-threshold memory cell arrays that
employ the circuit provided in the present invention can have higher
capacity and density.

[0027] Actually, when the supply voltage further decreases, the driving
power of a P-type transistor is significantly lower than that of an
N-type transistor (the minimum drain current of a P-type transistor is
approximately 22% of an N-type transistor under |Vds|=|Vgs|=200 mV, 0.13
μm process). To fully verify the pull-up capability of the present
invention, a comparison is made for sub-threshold memory cell arrays with
64 memory cells connected in series on the same bit line at 200 mV in the
worst case, by reading the potential information from the bit line in a
sub-threshold memory cell array that doesn't employ the circuit in the
present invention and a sub-threshold memory cell array that employs the
circuit in the present invention during "1" reading operation: the
potential information read from the bit line in the sub-threshold memory
cell array that employs the circuit in the present invention during "1"
reading operation is 117.1 mV, while the potential information read from
the bit line in the sub-threshold memory cell array that doesn't employ
the circuit in the present invention during "1" reading operation is
63.19 mV. Due to the pull-up effect of the circuit provided in the
present invention, the voltage loss on the read bit line during "1"
reading operation is reduced from 68.4% to 41.45%, which indicates the
circuit provided in the present invention can greatly alleviate the
problem of reading error resulting from the poor driving power of P-type
transistors in the reading operation, break through the limitation to the
number of memory cells permitted on the same bit line, and increase the
capacity and density of sub-threshold memory cell arrays.

[0028] According to the design described above, the present invention
doesn't require large-size transistors, and the unit density can be low.
Since a bit line can support more memory cells, memory cell arrays
implemented with the circuit provided in the present invention can have
higher capacity and higher overall density.